A conventional radio frequency (RF) receiver includes various gain stages for purposes of receiving a relatively small magnitude RF signal and translating the received RF signal to an intermediate frequency (IF) signal.
The RF signal that is received by the receiver typically is provided from an antenna source and varies in strength. An input stage of the receiver typically includes an amplifier such as a low noise amplifier (LNA) to amplify the incoming signal, preferably without introducing distortion. The LNA is typically designed to match the impedance of the source (e.g., antenna). In many instances, the LNA is designed to have a fixed impedance, typically 50 or 75 ohms (Ω).
Conventional amplifiers for a receiver are formed using bipolar complementary metal oxide semiconductor (BiCMOS) technology. For example, a common base amplifier formed of a bipolar transistor is often used. Such an amplifier has a transconductance, gm, that equals the bias current divided by 0.26 volts. Because the impedance seen by the source is approximately equal to 1/gm, the bias current is chosen to achieve the desired termination (i.e., matching) impedance. Furthermore, the gain of the amplifier is also linked to the transconductance and is equal to the transconductance multiplied by the load resistance. Accordingly, the power dissipation of a conventional LNA is determined when the input impedance is chosen. However, by using a matched impedance, power dissipation may be greater than desired.
In addition to power dissipation issues, a matched impedance LNA presents other problems, particularly in portable applications. Namely, it is difficult to control antenna impedance and therefore the source and amplifier may not have properly matched impedances, causing reflections. Furthermore, a single amplifier design cannot be flexibly used in multiple designs, as it cannot be accommodated to sources having different impedances.
To overcome at least the power dissipation issues, some designs use a resonant inductor-capacitor (LC) impedance transformation network. In such a design, the LNA input impedance may be set at a higher value, consuming less current in the amplifier, while still providing a matching impedance to the antenna. However, significant downsides to this approach exist. First, the transformation network requires large components which consume board real estate and add cost. Another concern is that the transformation network increases signal magnitude at the amplifier, which has a negative effect on intermodulation performance of the amplifier.
A need thus exists for improved amplifier designs for a receiver.